1. Technical Field
This invention relates to the field of semiconductor processing. More specifically, the invention relates to methods using disposable and permanent films to dope underlying layers through diffusion and using disposable films during implantation doping.
2. Background Art
When doping semiconductor layers, and in particular source and drain regions for Metal Oxide Semiconductor Field Effect Transistors (MOSFETs), ion implantation methods followed by an activation anneal are generally used. Ion implantation causes quite a bit of imperfections and defects in the silicon structure. To activate the implanted dopants, high temperature (>900° C.) annealing is performed. This allows the implanted dopant atoms to migrate into proper crystal structures. The higher the activation temperature, the more implanted dopants are activated. However, the defects created by ion implantation enhance the diffusion of dopants, and it is more difficult to form shallow and low resistivity source/drain diffusions.
This enhanced diffusion associated with ion implantation is particularly detrimental when forming extensions. Extensions are shallow doped areas, in the source and drain regions of MOSFETs, that connect the device channel to the deeper source and drain diffusions. When the MOSFET's size is reduced, it is beneficial to scale the extensions to as shallow as possible to reduce short channel effects (while still allowing silicide to be formed in the deeper source and drain diffusion regions), and to dope the extensions as high as possible to reduce device series resistance.
What is needed are methods for doping underlying semiconductor layers without enhancing the dopant diffusion during an activation anneal. These methods should also allow extensions to be formed with very thin extensions to provide better junctions, yet allow adequate control over the extensions' doping. Additionally, these methods should allow for layers used for doping that can subsequently be removed without damaging the remaining structure.